Some conventional fans with sound deadening boxes include a flow-dividing sound deadening member in the vicinity of their body air inlets. The flow-dividing sound deadening member means a sound deadening member having a corner with a recess on the opposite-motor-side casing-inlet-port-side air passage thereof. One such conventional fan is disclosed in Patent Document 1.
Other conventional fans with sound deadening boxes include an air guide plate in the vicinity of their casing inlet ports. One such conventional fan is disclosed in Patent Document 2.
One of the first-mentioned conventional fans with sound deadening boxes including a flow-dividing sound deadening member in the vicinity of their body air inlets will be described as follows with reference to drawings. FIGS. 26 and 27 show conventional fan 100 with a sound deadening box (hereinafter, fan 100) including the flow-dividing sound deadening member.
As shown in FIGS. 26 and 27, fan 100 is installed on the floor of attic 101. Fan 100 includes body 106, which is provided on its opposite sides with body air inlet 102 (hereinafter, inlet 102) and body air outlet 104 (hereinafter, outlet 104). Inlet 102 is disposed on the suction side of body 106 and connected to suction-side duct 103 (hereinafter, duct 103) so as to draw the air indoors 115 into body 106. Outlet 104 is disposed on the exhaust side of body 106 and connected to exhaust-side duct 105 (hereinafter, duct 105) so as to exhaust the air indoors 115 to outdoors 121 via body 106. Body 106 includes double inlet centrifugal fan 111 (hereinafter, fan 111).
Fan 111 includes fan casing 107 (hereinafter, casing 107), motor 108, opposite-motor-side casing inlet port 109 (hereinafter, port 109), and motor-side casing inlet port 110 (hereinafter, port 110). Fan 111 further includes double inlet impeller 120 (hereinafter, impeller 120) and disk-shaped main plate 119. Impeller 120 is disposed on both sides of main plate 119 and has blade inner diameter 118 substantially the same as the diameter of ports 109 and 110. Fan 100 further includes opposite-motor-side casing-inlet-port-side air passage 113 (hereinafter, air passage 113) on the inlet port 109 side and motor-side casing-inlet-port-side air passage 116 (hereinafter, air passage 116) on the inlet port 110 side. Air passage 113 is formed between inlet 102 and port 109, and air passage 116 is formed between inlet 102 and port 110. Fan 100 further includes flow-dividing sound deadening member 112, which is fixed on the side of casing 107 that faces inlet 102, that is, on the rear surface of casing 107. Flow-dividing sound deadening member 112 includes recess 114 on the air passage 113 side thereof so as to have a corner. Port 110 is disposed beyond motor outer diameter 117.
When fan 111 is driven, the air indoors 115 is drawn into body 106 through duct 103 and divided by flow-dividing sound deadening member 112. Of the divided air, an airflow passed through air passage 116 is drawn through port 110, and an airflow passed through air passage 113 is drawn through port 109. The airflows drawn through ports 109 and 110 pass through fan 111 and are exhausted to outdoors 121 through outlet 104 and duct 105.
In conventional fan 100, the presence of motor 108 causes air passage 116 to have a small width, and hence, a high wind speed. In order to prevent noise due to the high wind speed, the airflow in air passage 116 is controlled to be small. More specifically, flow-dividing sound deadening member 112 has recess 114 in the vicinity of the entrance air passage 113, making air passage 113 have a large passage section, and hence, a large airflow. Thus having a large airflow in air passage 113 divides the airflow into air passages 113 and 116 with a difference in air flow. Air passage 113 has a large passage section in the vicinity of inlet 102, but a small passage section in the vicinity of port 109, thereby causing a pressure loss in the vicinity of port 109 of air passage 113. As a result, conventional fan 100 has a high input, low static pressure, and a high level of airflow collision noise due to a narrow passage.
On the other hand, one of the second-mentioned fans with sound deadening boxes having an air guide plate in the vicinity of their casing inlet port will be described as follows with reference to FIGS. 28 to 30. FIGS. 28 to 30 show conventional fan 100a with a sound deadening box (hereinafter, fan 100a) having an air guide plate.
As shown in FIGS. 28 to 30, fan 100a is installed on the floor of attic 101. Fan 100a includes body 106, which is provided on its opposite sides with body air inlet 102 and body air outlet 104. Inlet 102 is disposed on the suction side of body 106 and connected to suction-side duct 103 so as to draw the air of indoors 115 into body 106. Outlet 104 is disposed on the exhaust side of body 106 and connected to exhaust-side duct 105 so as to exhaust the air indoors 115 to outdoors 121 via body 106. Body 106 includes single inlet centrifugal fan 111a (hereinafter, fan 111a).
Fan 111a includes fan casing 107, single inlet impeller 120a (hereinafter, impeller 120a), motor 108, and casing inlet port 109a (hereinafter, port 109a). Casing 107 includes scroll 137, casing side plate 138, discharge port 139, and tongue 140. Impeller 120a is disposed on disk-shaped main plate 119 and rotated in rotational direction 123 by motor 108. Fan 100a further includes air guide plate 148, which is inclined from intersection 146 of the center line of impeller 120a and body 106 toward outer periphery 147 of port 109a on the discharge port side.
Fan 100a further includes casing-inlet-port-side air passage 113a (hereinafter, air passage 113a), which is formed between inlet 102 and port 109a on the port 109a side. Air passage 113a has a width D smaller than the inner diameter of port 109a, and a center height at center height position 113b. Impeller 120a has a center height at center height position 120b. 
When fan 111a is driven, the air indoors 115 is drawn into body 106 through duct 103, and passes through air passage 113a. An airflow passed through air passage 113a is vertically divided along the outer periphery of scroll 137 into discharge-port-side airflow 121 (hereinafter, airflow 121) and tongue-side airflow 122 (hereinafter, airflow 122). The divided airflows are deflected by air guide plate 148 and drawn through port 109a. The airflows drawn through port 109a pass through fan 111a and are exhausted to outdoors 121 through duct 105.
In conventional fan 100a, air guide plate 148 is inclined with respect to port 109a in order to guide the airflow from air passage 113a through port 109a. Scroll 137, however, is generally logarithmic-spirally increased, causing center height position 120b of impeller 120a to be slightly lower than center height position 113b of air passage 113a. As a result, airflow 121 is larger than airflow 122. Airflow 121 swirls in rotational direction 123 of impeller 120a along the bottom of air guide plate 148, thereby being guided through port 109a. Airflow 122, on the other hand, swirls in rotational direction 123 of impeller 120a by the force of airflow 121 and becomes a pre-swirling flow to be guided through port 109a. 
Such a pre-swirling flow in the rotational direction of impeller 120a at port 109a reduces the relative speed of the airflow passing through rotating impeller 120a, thereby decreasing air flow. The pre-swirling flow also collides with the airflow from air passage 113a, thereby causing turbulent flow in the vicinity of port 109a. As a result, fan 100a has low air flow due to pressure loss, a high input, and a high level of turbulent flow noise.    Patent Document 1: Japanese Patent Unexamined Publication No. 2002-156139    Patent Document 2: Japanese Utility Model Unexamined Publication No. S61-119048